Programming method of controlling the amount of write current applied to phase change memory device and write driver circuit therefor

ABSTRACT

A programming method which controls the amount of a write current applied to a Phase-change Random Access Memory (PRAM), and a write driver circuit realizing the programming method. The programming method includes maintaining a ratio of a resistance of the PCM in the higher resistance state to a resistance of the PCM in the lower resistance state constant or substantially constant independent of an ambient temperature. The ratio may be maintained by increasing, decreasing, or keeping the same a reset current and/or a set current.

PRIORITY STATEMENT

This application claims priority of Korean Patent Application No.2003-58247 filed on Aug. 22, 2003, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a phase change memory device, and moreparticularly, to a programming method of controlling a write currentapplied to a phase change memory device according to the ambienttemperature and a write driver circuit realizing the programming method.

2. Description of the Related Art

A Phase-change Random Access Memory (PRAM) is a non-volatile memorydevice that stores data using a Phase Change Material (PCM), such asGe—Sb—Te (GST), whose phase and resistance change according to theambient temperature.

Information may be stored in the PRAM when the GST changes into acrystalline phase or an amorphous phase according to the ambienttemperature and/or the heating time. A high temperature, e.g., 900° C.or more, may applied to the PCM so as to change the phase of the PCM.The high temperature may be obtained from joule heating, generated fromcurrent flowing through a cell of the PRAM.

In a write operation, when a current is applied to the PCM to generate atemperature greater than or equal to its melting temperature and thenthe PCM is rapidly cooled down, the PCM becomes amorphous and data ‘1’may be recorded in the PCM. In this case, the PCM is said to haveentered a “reset” state.

If the PCM is heated at a temperature greater than or equal to acrystallization temperature, maintained for a given time, and cooleddown, the PCM becomes crystalline and data ‘0’ may be recorded in thePCM. In this case, the PCM is said to have entered a “set” state.

In a read operation, a bit line and a word line may be selected tochoose a particular memory cell. When an external current is applied tothe PCM, a resistance of the PCM changes. When the resistance of the PCMchanges, a voltage changes, thus allowing the expression of binaryvalues ‘1’ and ‘0’.

In a PRAM cell, the write operation may be affected by the ambienttemperature, since both the amount of a write current and a dynamicresistance of the PCM may change according to the ambient temperature.

In general, the higher the ambient temperature, the lower theperformance of a transistor that drives the write current. As thetransistor deteriorates, the amount of the write current and the dynamicresistance of the PCM also decrease.

Accordingly, thermal energy, i.e., temperature, which is generated fromjoule heating and is a direct cause of the phase change of the PCM cell,decreases, thus changing the GST into an unstable crystalline oramorphous state. Thus, the difference between a reset resistance and aset resistance becomes smaller and may cause errors during a readoperation.

FIG. 1 illustrates a variation in a reset resistance R_RESET or a setresistance R_SET according to the ambient temperature. Referring to FIG.1, a ratio of the reset resistance R_RESET to the set resistance R_SETdecreases as the ambient temperature increases. The ratio may decreasebecause the PCM does not reach a crystallization temperature or anamorphization temperature as the ambient temperature becomes higherduring the write operation.

Thus, as the ambient temperature becomes higher, a sensing marginbetween the reset state and the set state become smaller, thus causingpossible malfunction of a PRAM.

FIG. 2A illustrates a variation in the write current according to theambient temperature and FIG. 2B illustrates a variation in energy in thePCM according to the ambient temperature.

Referring to FIG. 2A, the higher the ambient temperature, the worse theperformances of the driver transistors, namely, the smaller the writecurrent applied to the PRAM cell.

In FIG. 2B, the energy denotes thermal energy that is generated from thejoule heating. The thermal energy generated from the joule heating maybe proportional to the square of a current flowing through the PRAM celland a resistance of the PCM. The higher the ambient temperature, thesmaller the current flowing through the PRAM cell, and the lower theresistance of the PCM. As shown, the thermal energy in the PRAM celldecreases as the ambient temperature increases.

When the PCM enters the reset state, the PCM is heated to a temperaturegreater than or equal to the melting temperature Tm and then changesinto an amorphous phase. However, if the ambient temperature exceeds agiven temperature, a reset current may not reach a desired reset currentrange and the temperature applied to the PRAM cell may be lower than themelting temperature, thus changing the PCM into an unstable amorphousstate.

Likewise, if a set current does not fall within a desired set currentrange, the temperature applied to the PRAM cell may not reach acrystallization temperature Tc, thus changing the PCM into an unstablecrystalline state.

As a result, when the temperature applied to the PCM is lower than themelting temperature Tm, the PCM does not completely change into theamorphous state. When the temperature applied to the PCM is lower thanthe crystallization temperature Tc, the PCM does not completely changeinto the crystalline state. In both cases, both the reset resistance andthe set resistance are lowered during the read operation, which maycause an operation error, where the reset state may be erroneouslydetected as the set state.

SUMMARY OF THE INVENTION

Example embodiments of the present invention provides a programmingmethod which controls a write current applied to a Phase-change RandomAccess Memory (PRAM) according to the ambient temperature.

Example embodiments of the present invention also provides a writedriver circuit which controls a write current applied to a PRAM cellaccording to the ambient temperature.

An example embodiment of the present invention is directed to aprogramming method used in a phase-change random access memory (PRAM)which uses a phase change material (PCM) that changes into a higherresistance state or a lower resistance state in response to a currentpulse, including increasing the amount of a reset current when changingthe PCM into the higher resistance state and the amount of a set currentwhen changing the PCM into the lower resistance state when the ambienttemperature increases.

In an example embodiment of the present invention, the amounts of thereset current and the set current are increased when the ambienttemperature increases, so as to keep a ratio of a resistance of the PCMin the higher resistance state to a resistance of the PCM in the lowerresistance state constant regardless of the ambient temperature.

In an example embodiment of the present invention, the amount of thereset current is increased to an amount within a range in which the PCMchanges into the higher resistance state and the amount of the setcurrent is increased to an amount within a range in which the PCMchanges into the lower resistance state.

Another example embodiment of the present invention is directed to aprogramming method used in a PRAM which uses a PCM that changes into ahigher resistance state or a lower resistance state in response to acurrent pulse, including increasing the amount of a reset current whenthe ambient temperature increases, so as to change the PCM into thehigher resistance state.

In an example embodiment of the present invention, the programmingmethod may further include reducing the amount of a set current when theambient temperature increases, so as to change the PCM into the lowerresistance state.

In an example embodiment of the present invention, the amount of the setcurrent is reduced to an amount within a range in which the PCM changesinto the lower resistance state.

In an example embodiment of the present invention, when the ambienttemperature increases, the amount of the reset current increases and theamount of the set current decreases so as to increase a ratio of aresistance of the PCM in the higher resistance state to a resistance ofthe PCM in the lower resistance state.

Another example embodiment of the present invention is directed to awrite driver circuit included in a PRAM which uses a PCM that changesinto a higher resistance state or a lower resistance state in responseto a current pulse. The write driver circuit may include a currentcontroller unit which controls generation of a reset current or a setcurrent in response to a write active signal, and data or inversed data;and a current driver unit which is controlled by the current controllerto generate the reset current or the set current and changes the amountof the reset current or the set current in response to a positivecontrol signal.

In an example embodiment of the present invention, the positive controlsignal is a voltage signal whose voltage level increases when theambient temperature increases. In an example embodiment of the presentinvention, the current driver unit may include a current mirror unitwhich outputs the reset current or the set current and a positivecontroller which increases the amount of the reset current or the setcurrent in response to the positive control signal.

In an example embodiment of the present invention, the current mirrorunit may include a first transistor having a first terminal connected toa line voltage, and a gate and a second terminal which are connected toa first node; and a second transistor having a first terminal connectedto the line voltage, a gate connected to the first node, and a secondterminal which outputs the reset current or the set current.

In an example embodiment of the present invention, the currentcontroller unit may include a third transistor having a first terminalconnected to the first node, a gate to which the data is applied, and asecond terminal connected to a second node; a fourth transistor having afirst terminal connected to the first node, a gate to which the inverseddata is applied, and a second terminal connected to the second node; anda fifth transistor having a first terminal connected to the second node,a gate to which the write active signal is applied, and a secondterminal connected to a ground voltage.

In an example embodiment of the present invention, the positivecontroller may be a sixth transistor having a first terminal connectedto the first node, a gate to which the positive control signal isapplied, and the second terminal connected to the second node.

In an example embodiment of the present invention, the current driverunit may further include a seventh transistor having a first terminalconnected to the first node, a gate to which a negative control signalis applied, and a second terminal connected to the second node.

In an example embodiment of the present invention, the negative controlsignal may be a voltage signal whose voltage level decreases when theambient temperature increases.

In an example embodiment of the present invention, the write drivercircuit may further include a thermal compensator which generates thepositive control signal and the negative control signal according to theambient temperature. In an example embodiment of the present invention,the thermal compensator may include a band gap reference circuit.

Another example embodiment of the present invention is directed to aprogramming method used in a PRAM which uses a PCM that changes into ahigher resistance state or a lower resistance state in response to acurrent pulse, including maintaining a ratio of a resistance of the PCMin the higher resistance state to a resistance of the PCM in the lowerresistance state constant or substantially constant independent of anambient temperature.

In an example embodiment of the present invention, the ratio may bemaintained by increasing, decreasing, or keeping the same a resetcurrent and/or a set current.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent by describing in detailexemplary embodiments thereof with reference to the attached drawings inwhich:

FIG. 1 is a graph illustrating variations in a reset resistance and aset resistance according to the ambient temperature;

FIG. 2A is a graph illustrating variations in write currents accordingto the ambient temperature;

FIG. 2B is a graph illustrating variations in energy in a phase changematerial (PCM) according to the ambient temperature;

FIG. 3 is a graph illustrating variations in the reset resistance andthe set resistance according to the ambient temperature when using aprogramming method according to an exemplary embodiment of the presentinvention;

FIG. 4A is a graph illustrating variations in the write currentsaccording to the ambient temperature when using a programming methodaccording to an exemplary embodiment of the present invention;

FIG. 4B is a graph illustrating variations in energy in the PCMaccording to the ambient temperature when using a programming methodaccording to an exemplary embodiment of the present invention;

FIG. 5A is a graph illustrating variations in the reset resistance andthe set resistance according to the ambient temperature when using aprogramming method according to an embodiment of the present invention;

FIG. 5B is a graph illustrating variations in the write currentaccording to the ambient temperature when using a programming methodaccording to an exemplary embodiment of the present invention;

FIG. 6 is a circuit diagram of a write driver circuit that can implementa programming method according to an exemplary embodiment of the presentinvention;

FIG. 7 is a graph illustrating a positive control signal and a negativecontrol signal according to exemplary embodiments of the presentinvention; and

FIG. 8 is a circuit diagram of a write driver circuit using a negativecontrol signal according to an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed in a greater detail with reference to the accompanyingdrawings. The same reference numerals represent the same elementsthroughout the drawings.

A programming method according to an exemplary embodiment of the presentinvention, which may be used in a Phase-change Random Access Memory(PRAM), may include increasing the amount of a reset current to change aPhase Change Material (PCM) into a higher resistance state and/orincreasing the amount of a set current to change the PCM into a lowerresistance state when the ambient temperature increases.

In other words, in the PRAM cell according to an embodiment of thepresent invention, the amount of the set current and/or the resetcurrent may be increased when the ambient temperature increases. Theamount of the reset current and/or the set current may be increased suchthat a ratio of a resistance of the PRAM in the higher resistance stateto that of a resistance in the lower resistance state is maintainedregardless of the ambient temperature.

There are several ways to increase the amount of current. For instance,a write current may be controlled whenever the ambient temperatureincreases, using a circuit that senses an increase in the ambienttemperature. An example of controlling the write current will bedescribed below in more detail.

In an example embodiment, the amount of reset current may be increasedto an amount that lies within a desired range in which the PCM changesinto a higher resistance state, and/or that of the set current may beincreased to an amount that lies within a desired range in which the PCMchanges into a lower resistance state.

FIG. 3 is a graph illustrating variations in a reset resistance R_RESETand a set resistance R_SET according to the ambient temperature whenusing a programming method according to an exemplary embodiment of thepresent invention. Referring to FIG. 3, a ratio of the reset resistanceR_RESET and the set resistance R_SET may be maintained regardless of theambient temperature. Thus, during a read operation, it is possible toachieve a more stable sensing operation irrespective of the ambienttemperature and/or reduce or prevent the occurrence of an error in theread operation.

FIG. 4A is a graph illustrating variations in the write currentsaccording to the ambient temperature when using the programming methodaccording to an exemplary embodiment of the present invention. FIG. 4Bis a graph illustrating a variation of energy in the PCM according tothe ambient temperature when using the programming method according toan exemplary embodiment of the present invention.

In the programming method according to an exemplary embodiment, theamounts of the reset current I_RESET and/or the set current I_SET areincreased when the ambient temperature increases. As a result, thermalenergy applied to the PRAM cell can be maintained regardless of theambient temperature, as shown in FIG. 4B.

FIG. 5A is a graph illustrating variations in the reset resistanceR_RESET and the set resistance R_SET according to the ambienttemperature when using a programming method according to anotherexemplary embodiment of the present invention. FIG. 5B is a graphillustrating variations in write currents according to the ambienttemperature when using a programming method according to anotherexemplary embodiment of the present invention.

Referring to FIG. 5B, the amount of the reset current I_RESET may beincreased when the ambient temperature increases, thereby maintainingthe reset resistance R_RESET regardless of the ambient temperature.

The amount of the set current I_SET may be reduced when the ambienttemperature becomes higher, thus causing a reduction in the setresistance R_SET. As a result, a ratio of the reset resistance R_RESETto the set resistance R_SET may be increased as the ambient temperaturebecomes higher, thereby improving a sensing margin during a readoperation. An increase in the ratio is illustrated by (i) and (ii) ofFIG. 5A.

As shown in FIG. 5B, the amount of the set current I_SET may be reducedto an amount that still lies within a desired range in which the PCM canchange into the lower resistance state.

If the amount of the set current I_SET does not fall within the range,the PCM does not change into the crystallization state, and therefore,an amount of the set current I_SET may be reduced within the desiredrange when the ambient temperature increases.

As described above, in exemplary embodiments of the present invention,the ratio of the reset current I_RESET and set current I_SET may bemaintained when the ambient temperature increases by increasing,decreasing, or keeping the same the amounts of the reset current I_RESETand/or the set current I_SET.

FIG. 6 is a circuit diagram of a write driver circuit 600 that canimplement a programming method according to exemplary embodiments of thepresent invention. Referring to FIG. 6, the write driver circuit 600 mayinclude a current driver unit 610 and a current controller unit 640.

The current controller unit 640 may control generation of the resetcurrent I_RESET or the set current I_SET in response to a write activesignal WEN, and data DATA or reversed data/DATA. The current driver unit610 may be controlled by the current controller unit 640 to generate thereset current I_RESET and/or the set current I_SET, and changes theamount of the reset current I_RESET or the set current I_SET in responseto a control signal, such as positive control signal PTEM.

FIG. 7 is a graph illustrating the positive control signal PTEM and anegative control signal NTEM. The positive control signal PTEM may be avoltage signal whose voltage level increases when the ambienttemperature increases.

Hereinafter, the operation of a write drive circuit according to anotherexemplary embodiment of the present invention will be described withreference to FIGS. 6 and 7. The current driver unit 610 may include acurrent mirror unit 620 and a positive controller 630. The currentmirror unit 620 may output the reset current I_RESET or the set currentI_SET.

More specifically, the current mirror unit 620 may include a firsttransistor TR1 and a second transistor TR2. The first transistor TR1 mayhave a first terminal connected to a line voltage VDD, and a secondterminal and a gate that are connected to a first node N1. The secondtransistor TR2 may have a first terminal connected to the line voltageVDD, a second terminal connected to the first node N1, and a gateconnected to the first node N1. Also, the second transistor TR2 mayoutput the reset current I_RESET or the set current I_SET.

The current controller unit 640 may include a third transistor TR3, afourth transistor TR4, and a fifth transistor TR5. The third transistorTR3 may have a first terminal connected to the first node N1, a gate towhich the data DATA is applied, and a second terminal connected to asecond node N2. The fourth transistor TR4 may have a first terminalconnected to the first node N1, a gate to which the reversed data /DATAis applied, and a second terminal connected to the second node N2. Thefifth transistor TR5 may have a first terminal connected to the secondnode N2, a gate to which the write active signal WEN is applied, and asecond terminal connected to a ground voltage VSS.

The third transistor TR3 may be larger than the fourth transistor TR4.Thus, when the third and fifth transistors TR3 and TR5 are turned on, avoltage level of the first node N1 may be lower than when the fourth andfifth transistors TR4 and TR5 are turned on.

When the write active signal WEN is applied at a first level (forexample, a “high” level) to the gate of the fifth transistor TR5, thecurrent controller unit 640 starts to operate. The level of the writeactive signal WEN applied need not be limited, that is, the currentcontroller unit 640 may be set to operate in response to the writeactive signal WEN at a low level.

Assuming the data DATA and the reversed data /DATA are applied at afirst (high) level and a second (low) level so as to write data ‘1’ to aPRAM cell (not shown), i.e., the PRAM cell enters a reset state. In thiscase, the third transistor TR3 may be turned on, but the fourthtransistor TR4 may be turned off.

When the third and fifth transistors TR3 and TR5 are turned on, avoltage level of the first node N1 may decrease, and as a result, thefirst and second transistors TR1 and TR2 may be turned on. Since thefirst and second transistors TR1 and TR2 form a current mirror, theamount of current output from the second terminal of the secondtransistor TR2 depends on a ratio of the first transistor TR1 to thesecond transistor TR2.

Application of the data DATA at a first (high) level may cause the thirdtransistor TR3 to be turned on and the reset current I_RESET to beoutput from the second transistor TR2. However, if the ambienttemperature increases, the performances of the transistors TR1 and TR2of the current driver unit 610 may deteriorate, and as a result, theamount of the reset current I_RESET output from the transistors TR1 andTR2 may decrease as indicated with reference to FIG. 2A.

The write driver circuit 600 may further include the positive controller630. The positive controller 630 may increase the amount of the resetcurrent I_RESET or the set current I_SET in response to the positivecontrol signal PTEM.

The positive controller 630 may include a sixth transistor TR6 includinga first terminal connected to the first node N1, a gate to which thepositive control signal PTEM is applied, and a second terminal connectedto the second node N2. As shown in FIG. 7, the positive control signalPTEM may be a voltage signal whose voltage level increases when that theambient temperature increases.

When the performances of the transistors TR1 and TR2 is degraded due toan increase in the ambient temperature, the positive control signal PTEMmay turn on the sixth transistor TR6 and a voltage level of the firstnode N1 may decrease.

If the ambient temperature continues to increase, the voltage level ofthe positive control signal PTEM may also continues to increase and thevoltage level of the first node N1 may decrease. In this case, theamount of the reset current I_RESET output from the second transistorTR2 increases.

The write drive circuit 600 may further include a thermal compensator650 that generates the positive control signal PTEM in accordance withthe ambient temperature. As an example, the thermal compensator 650 mayuse a band gap reference circuit.

The band gap reference circuit may sense the ambient temperature, andoutput a signal whose voltage level increases and a signal whose voltagelevel decreases when the ambient temperature increases. Band gapreference circuits are well known to those skilled in the art and adetailed description thereof will be omitted.

The positive control signal PTEM generated by the thermal compensator650 may increase the amount of the reset current I_RESET when theambient temperature increases. As illustrated in FIG. 4B, it is possibleto maintain thermal energy for the phase of the PCM regardless of theambient temperature by controlling the amount of the reset currentI_RESET according to the ambient temperature.

The resistivity of the PCM may also be maintained regardless of theambient temperature. In example embodiments above, generation of thereset current I_RESET when the data DATA is at a high level is describedfor convenience, but the set current I_SET may also be generated usingthe same techniques as when generating the reset current I_RESET, usingthe write drive circuit 600. Therefore, generation of the set currentI_SET will not be described.

FIG. 8 is an example circuit diagram of a-write driver circuit 800 usinga negative control signal NTEM. Referring to FIG. 8, compared to thewrite driver circuit 600, the write driver circuit 800 may furtherinclude a seventh transistor TR7 that is controlled by the negativecontrol signal NTEM. More specifically, a current driver unit 810 of thewrite driver circuit 800 may further includes the seventh transistor TR7having a first terminal connected to a first node N1, a gate to whichthe negative control signal NTEM is applied, and a second terminalconnected to a second node N2.

The negative control signal NTEM may be a voltage signal whose voltagelevel decreases when the ambient temperature increases. The negativecontrol signal NTEM may be output from a thermal compensator 850.

The seventh transistor TR7 may be a PMOS transistor that reduces avoltage level of the first node N1 in response to the negative controlsignal NTEM when the ambient temperature increases. For this reason, theamount of a current output from a second transistor TR2 may increasewhen the ambient temperature increases.

Also, regardless of the ambient temperature, thermal energy for changingthe phase of the PCM can be maintained as shown in FIG. 4B and/or theresistivity of the PCM can be maintained as shown in FIG. 3.

As described above, a programming method and a write driver circuit of aPRAM according to exemplary embodiments of the present invention mayincrease the amount of a write current applied to a PRAM cell when theambient temperature increases. Accordingly, it is possible to maintain aratio of a reset resistance to a set resistance and maintain asufficient sensing margin during a read operation.

Although exemplary embodiments of the present invention have beendescribed as utilizing a ‘high’ logic state, and a ‘low’ logic state, itwill be understood that any logic state may be utilized as may bedesired by one of ordinary skill in the art.

Although exemplary embodiments of the present invention have beendescribed with specific circuits implementations, it will be known thatany other implementations may also be used as desired by one of ordinaryskill in the art.

In an example embodiment, the PCM may include at least one of sulfur(S), selenium (Se), and tellurium (Te). In an example embodiment, thePCM may include at least one chalcogenide. In an example embodiment, thePCM may be of at least one Group VB element, including tantalum (Ta),niobium (Nb), and vanadium (V) and antimony-tellurium (Sb—Te), or atleast one a Group VIB element including tungsten (W), molybdenum (Mo)and chromium (Cr) and antimony-selenium (Sb—Se). In an exampleembodiment, the PCM may include germanium-antimony-tellurium (GST). Inan example embodiment, the PCM further includes at least one impurity.In an example embodiment, the at least one impurity may be nitrogen.

While exemplary embodiments of the present invention have beenparticularly shown and described with reference to embodiments thereof,it will be understood by those skilled in the art that various changesin form and details may be made therein without departing from thespirit and scope of the invention as defined by the appended claims.

1-26. (canceled)
 27. A write driver circuit included in a phase-changerandom access memory (PRAM) which uses a phase change material (PCM)that changes into a higher resistance state or a lower resistance statein response to a current pulse, the write driver circuit comprising: acurrent controller unit which controls generation of a reset current inresponse to a write active signal, and data or inversed data; and acurrent driver unit which is controlled by the current controller togenerate the reset current and changes the amount of the reset currentin response to a positive control signal.
 28. The write driver circuitof claim 27, wherein the positive control signal is a voltage signalwhose voltage level increases when the ambient temperature increases.29. The write driver circuit of claim 27, wherein the current driverunit comprises: a current mirror unit which outputs the reset current;and a positive controller which increases the amount of the resetcurrent in response to the positive control signal.
 30. The write drivercircuit of claim 29, wherein the current mirror unit comprises: a firsttransistor having a first terminal connected to a line voltage, and agate and a second terminal which are connected to a first node; and asecond transistor having a first terminal connected to the line voltage,a gate connected to the first node, and a second terminal which outputsthe reset current.
 31. The write driver circuit of claim 30, wherein thecurrent controller unit comprises: a third transistor having a firstterminal connected to the first node, a gate to which the data isapplied, and a second terminal connected to a second node; a fourthtransistor having a first terminal connected to the first node, a gateto which the inversed data is applied, and a second terminal connectedto the second node; and a fifth transistor having a first terminalconnected to the second node, a gate to which the write active signal isapplied, and a second terminal connected to a ground voltage.
 32. Thewrite driver circuit of claim 31, wherein the positive controller is asixth transistor having a first terminal connected to the first node, agate to which the positive control signal is applied, and the secondterminal connected to the second node.
 33. The write driver circuit ofclaim 32, wherein the current driver unit further comprises a seventhtransistor having a first terminal connected to the first node, a gateto which a negative control signal is applied, and a second terminalconnected to the second node.
 34. The write driver circuit of claim 33wherein the negative control signal is a voltage signal whose voltagelevel decreases when the ambient temperature increases.
 35. The writedriver circuit of claim 34, further comprising a thermal compensatorwhich generates the positive control signal and the negative controlsignal according to the ambient temperature.
 36. The writer drivercircuit of claim 35, wherein the thermal compensator includes a band gapreference circuit.
 37. A phase-change random access memory (PRAM) whichuses a phase change material (PCM) that changes into a higher resistancestate or a lower resistance state in response to a current pulse,comprising: a thermal compensator which generates a control signalaccording to the ambient temperature; and a writer driver to generate areset current and change the amount of the reset current in response tothe control signal.